1. Field of Invention
The present invention relates to a method for data storage and a unit using the same, and more particularly to a method for data storage of a memory unit and a memory unit using the same.
2. Description of Related Art
Along with the rapid development of science and technology at present, for storing a large amount of data, memory is being developed to have larger capacity, higher speed, longer service life, and being more power-saving. Generally, memory with a capacity of hundreds of M (megabytes) already cannot meet the demands, and memory with a capacity of several G (gigabytes) or hundreds of G (gigabytes) has appeared. However, the property of high capacity of the memory may easily cause the voltage distribution range of data storage in the memory to become excessively large. Thus, there are two defects of the conventional art. First, to increase the sensing window between each state (for example, the first state is set to logic level “1” and the second state is set to logic level “0”) to enhance the sensing effect, the program threshold voltage difference between each state must be increased, which results in more power consumption and shorter service life. Second, to reduce the program threshold voltage difference between each state to prolong the service life and reduce the power consumption of the memory, the sensing window between each state is thus reduced, which results in difficulty to perform the sensing process for distinguishing each state. The following part will give an illustration with reference to FIGS. 1 and 2.
FIG. 1 is a threshold voltage distribution diagram of the data storage operation of a conventional memory unit. The horizontal axis represents the threshold voltage, and the longitudinal axis represents the number of bits. A threshold voltage distribution curve 101 and a threshold voltage distribution curve 102 represent distribution curves for a memory of 1M to store data in the first state and the second state. A threshold voltage distribution curve 103 and a threshold voltage curve 104 represent distribution curves for a memory of 1 G to store data in the first state and the second state. SW11 indicates the sensing window between the two states of a two-bits memory, SW12 and SW13 indicate the sensing window between two states of the memories of 1M memory and 1 G memory. DVt11 indicates the delta threshold voltage after program between the two states of a two-bits memory, DVt12 and DVt13 indicate the delta threshold voltage after program between two states of the memories of 1M memory and 1 G memory. Seen from FIG. 1, the distribution value D2 of the curve 103 is larger than the distribution value D1 of the curve 101. The two-bits memory has only one threshold voltage point, and thus the distribution value thereof is nearly zero. As shown in FIG. 1, it is presumed that every bit has the same program delta threshold voltage (DVt). Generally, the sensing window is defined as the threshold voltage difference between a high boundary of the low threshold voltage distribution and a low boundary of the high threshold voltage distribution, and the program delta threshold voltages for the two-bits memory such as 1M memory or 1 G memory are DVt11, DVt12 and DVt13 and are presumed to be the same, the sensing window voltage are different because the relationship between the sensing window and the program delta threshold voltage is SW11=DVt11, SW12=(DVt12−D1), and SW13=(DVt13−D2) respectively. To keep the program threshold voltage difference of each memory as the same, the sensing window SW13 of the high-capacity memory becomes smaller, thus making it difficult to distinguish the two states of the high-capacity memory.
FIG. 2 is a threshold voltage distribution diagram of the data storage operation of another conventional memory unit. The horizontal axis represents the threshold voltage, and the longitudinal axis represents the number of bits. A threshold voltage distribution curve 201 and a threshold voltage distribution curve 202 represent distribution curves for a 1M memory to store data in the first state and the second state, a threshold voltage distribution curve 203 and a threshold voltage distribution curve 205 represent distribution curves for a 1 G memory to store the data in the first state and the second state, and threshold voltage distribution curves 204, 206 represent distribution curves of a part of the bits in the memory of 1 G in the two states. The sensing windows of the two-bits 1M memory and 1 G memory are SW11, SW12 and SW13 respectively. The program threshold voltage differences between the two states of the two-bits 1M memory and 1 G memory are DVt21, DVt22 and DVt23 respectively. The relationships between the sensing window differences and the program threshold voltage differences are SW21=DVt21, SW22=(DVt22−D1), and SW23=(DVt23−D2). To keep the sensing window of each memory as the same, the program threshold voltage difference DVt23 between two states of the high-capacity memory is increased, thereby shortening the service time and increasing the power consumption.